The present invention relates to steam traps, and in particular the present invention relates to float-type steam traps in which a float is connected with a valve member by a lever.
Many prior art float-type steam traps have been built in which a valve member is connected to a pivotable lever to move the valve member into and out of engagement with an orifice in the valve seat. It has previously been recognized that it is necessary to provide a relatively high mechanical advantage between the float and the valve member to make the initial movement of the valve member away from the orifice against the pressure of steam within the trap. To achieve this the distance between the pivot point of the lever and the valve member has been minimized while the distance between the pivot point of the lever and the float has been maximized. Lengthening the later distance means that the overall size of the trap including its relatively massive housing must be increased. Obviously, a steam trap with a high mechanical advantage but a small size is more economical to produce and lighter and therefore easier to install.
Moreover, instead of having a hinged connected between the lever and the valve seat, it has proved advantageous to permit the valve member to be self-centering in the valve seat. To accomplish this the lever is provided with one or more fulcrums about which it can pivot, and the fulcrums pivot against a flat surface which is fixed with respect to the valve seat. A bracket holds the lever and valve member relatively near the valve seat and keeps the lever, valve member, and float from falling down in the trap when the valve is wide open. With this arrangement the valve member can be self-centering in the valve orifice.
In several prior art devices the lever has been provided with two fulcrums, one to provide high mechanical advantage for initial movement of the valve member away from the valve seat and another to provide for subsequent rapid movement of the valve member. These dual fulcrum mechanisms can open a valve upon only a slight change in the bouyancy of the bucket because of the high initial mechanical advantage. After the initial movement, large forces are not required to move the valve further open. The dual fulcrum mechanisms thus effect early and rapid complete opening of the valve. This reduces errosion of the valve seat known as wire draw which is caused by high speed steam flowing through a small opening.
In one of the dual fulcrum devices the fulcrum for high mechanical advantage pivots on a surface of a bracket which is secured to the valve seat. The pivot surface is in a plane which is parallel with the orifice in the valve seat and which includes a diameter of the hemispherical valve member when the valve member is seated in the orifice. This trap has a very high mechanical advantage for initial opening of the trap because the distance between the fulcrum and the centerline of the valve seat is relatively small. Such a device is disclosed in U.S. Pat. No. 2,637,335 and is illustrated schematically in FIG. 2.
Although this type of steam trap operates satisfactorily, it is necessarily expensive and difficult to produce. Parts must be manufactured within tight tolerances if the pivot surface is to be properly located. A deviation from ideal dimensions in the bracket can combine with a deviation from ideal dimensions in the valve seat to result in a steam trap which is not marketable. The cumulative effect of "stackup" of inaccuracies can be overcome only by holding very tight tolerances in the manufacture of each component, and this is costly.
Another prior art device (illustrated in FIG. 1) uses a dual fulcrum lever which pivots on a plane which defines the valve orifice. This arrangement eliminates the stack up of tolerances problem, but also limits the maximum mechanical advantage obtainable. If the high leverage fulcrum is to pivot on the plane of the orifice, it cannot be located closer to the centerline of the valve seat than a distance equal to the radius of the hemispherical valve member. If the fulcrum is moved closer than that minimum, valuable movement of the float is wasted because it merely causes the valve member to rotate in the valve orifice rather than lift away from it.